Page:Aerial Flight - Volume 2 - Aerodonetics - Frederick Lanchester - 1908.djvu/29

Rh aeroplane—(1) an equilibrium between the attitude of the plane and the direction of flight, that is to say, the plane tends to preserve a predetermined angle to its line of travel; (2) equilibrium of a dynamical kind between the kinetic and potential energy of the aeroplane, by which constancy of gliding angle and velocity is secured.

It may be stated at once that the same principles apply quite generally to aerodones of more fully developed type, in fact, so far as the author is aware, there is no other way by which longitudinal stability can be automatically secured, and any aerodone to fly successfully must comply with the necessary conditions.

'''§ 5. The Ballasted Aeroplane. Lateral Stability.'''—The lateral stability of the ballasted aeroplane is closely connected with the

Fig. 4.

square-cut form of its extremities; it is at least certain that an oval or an elliptical plan form is relatively deficient in this respect.

It is evident that so long as a rectangular plane is travelling in a direction parallel to its end edges, the sustaining forces are symmetrical, and there is no turning moment about the axis of flight. If from any accidental cause, such as a slight motion of the air, the plane finds itself moving so that its ends make an angle with the line of flight (Fig. 4), the leading end L will, so